A. Field of the Invention
This invention relates to a method of removing iron and copper containing scale from a metal surface such as a ferrous metal surface whereby copper is not replated on the metal surface and the formation of undesirable precipitates is avoided.
B. Description of the Prior Art
In steam boilers, petrochemical process equipment, feed water heaters and associated piping, and in various types of pressure vessels, such as high pressure steam generating equipment utilized in electric power generation and other applications, in which water is circulated and heat transfer occurs, water-insoluble salts deposit as scale on the metallic interior surfaces. The formation or depositation of scale markedly reduces the heat transfer through the walls of such pipes or vessels and furthermore, the capacity of the pipes or vessels and passage of fluids therethrough is restricted by the formation of such scale. Because the scale formation on the inner surfaces of the pipes, particularly in heat transfer applications, the pipes are subject to excessive heat due to the loss of the heat transfer capacity. Furthermore, great pressures are required to overcome the restricting effect of the deposited scale. These disadvantages often lead to leaks and ruptures which necessitate undesirable down time and maintenance costs. Still further, the reduced cross-section of the pipes caused by the formation of the scale can cause an increased pressure drop.
The scale deposit on the vessels many times contains oxides of various metals such as iron oxides, e.g. magnetite and hematite. In addition, the scale deposit on the vessels can contain deposits of other iron salts. Furthermore, many times vessels are associated with equipment such as condensers, that are constructed, in part, from alloys of copper, and the scale produced on the inner surface of the vessel is frequently found to contain copper scale as well, in the form of elemental copper and sometimes copper oxides such as cuprous and cupric oxides. In addition, the copper scale can contain other copper salts such as cuprous phosphate.
Many types of methods have been proposed for removing copper and iron-containing scale from a metal surface, such as a ferrous metal surface. One method of removing copper and iron-containing scale from a ferrous metal surface involves the use of a strong mineral acid, particularly hydrochloric acid, for purposes of dissolving the scale found on the surface of the process equipment, boilers, feedwater heaters, and other types of vessels. Although utilization of corrosion inhibitors with mineral acids for cleaning operations has generally prevented acid attack on the equipment surface, such acid treatments have not been universally successful because, when the scale contains copper or copper oxides, many times copper is not dissolved during the cleaning operation and such copper that is dissolved is redeposited as elemental copper throughout the interior of the surfaces of the vessel. Such copper redepositation serves to accelerate corrosion and to interfere with heat exchange reactions.
U.S. Pat. Nos. 3,854,996 and 3,996,062, which are assigned to the assignee of the present invention and are hereby incorporated by reference, disclose the use of polyphosphonic acid or the alkali metal or amine salt of this acid to remove copper and iron-containing scale from a ferrous metal surface. Although the process described in these patents has met with commercial success, the use of a composition containing, principally, polyphosphonic acid or the amine or alkali metal salts of this acid, presents certain difficulties. For instance, during a cleaning process utilizing the polyphosphonic acid or its salts, an insoluble curdy, adhesive precipitate can be formed which may damage the equipment sought to be cleaned. The formation of the precipitate is of particular significance when iron content of the cleaning solution exceeds 0.9 weight percent of the cleaning solution.
Another proposed procedure for removing copper containing iron oxide scale from metal surfaces comprises contacting the metal surfaces with an aqueous, alkaline, solution containing an ammonium, amine, or hydroxy alkylamine salt of nitrilotriacetic acid, N-2-hydroxyethylamino diacetic acid, or an alkylenepolyamine aminopolycarboxylic acid. The use of this composition for removing iron and copper containing scale from a metal surface can result in the formation of an insoluble precipitate which adheres to the metal surfaces being cleaned. These precipitates must be removed from the metal surface either by mechanical means or by a different chemical removal treatment. It is believed that the precipitates can be prevented by maintaining in the cleaning solution an amount of chelating agent in excess of the amount theoretically required to complex the dissolved metals. This procedure, however, requires close analytical control of the cleaning solution and also increases the expense of the cleaning operation as a greater than stoichiometric amounts of the chelant must be maintained in the cleaning composition.
It is therefore desirable to provide a method of removing copper and iron containing scales from a metal surface, such as a ferrous metal surface, which is dependable, does not form undesirable precipitates, and which does not result in the redepositation of copper during the cleaning operation.